The present invention relates to a dual string orbital drilling system for drilling bore holes in earth formations. The present invention has significant advantages when drilling through fractured rock.
Generally bore holes are formed by rotating a drill string which may be several hundred feet in length into an earth formation. Dual wall drilling systems are known. For example, U.S. Pat. No. 4,618,172 to Becker shows a dual wall drill pipe having an outer pipe with a drill bit mounted at the drilling end of the outer pipe and an inner pipe used for providing a passageway for returning drilling fluid directed to the drill bit returning the drilling fluid to the surface. U.S. Pat. No. 4,940,098 to Moss shows another dual wall drilling system where the inner pipe is used for returning drilling fluid to the surface and a drill bit is mounted at the drilling end of the outer pipe.
A need exists, however, for a dual string drilling system where both the outer drilling string and an inner drilling string are used together to drill a bore hole.
A need also exists for a drilling system which allows an inner drilling string to perform drilling operations while being supported by an outer drill string which then allows the outer drill string and the inner drill string to be of reduced diameter thereby enabling a user to use the drilling system for drilling small holes approximately 3″ in diameter for long depths such as 10,000 feet.